1. Field of the Invention
The present invention relates to a power supply device that controls the connection state between a load and a battery pack that is composed of a plurality of rechargeable batteries. In particular, the present invention relates to a controller that controls the connection state between a battery pack and an inverter in electric vehicles. Examples of such electric vehicles can be provided by battery electric vehicle (BEV), hybrid electric vehicle (HEV), hybrid electric vehicle including fuel cells and rechargeable batteries. Such electric vehicles convert DC electric power into AC electric power by means of an inverter, and provide the converted electric power to an electric motor. The DC electric power is provided from a battery pack as a driving power source installed on the electric vehicles.
2. Description of the Related Art
In recent years, in consideration of environmental issues, hybrid electric vehicles have been commercially practical that are driven by an efficient combination of internal-combustion engine and electric motor. Hybrid electric vehicles include a rechargeable battery pack. The battery pack provides electric power to the electric motor in start, acceleration and the like, and retrieves vehicle kinetic energy as electric power in downhill, breaking and the like.
A hybrid electric vehicle has been proposed that includes a configuration that allows the included battery pack to be charged with electric power provided from external power sources such as commercial power. In this hybrid electric vehicle, in the case where the battery pack is previously charged by external power sources, the vehicle can be driven for a relatively short distance, such as in driving to and from work or for shopping, with its internal-combustion engine being stopped. As a result, it is possible to improve the total fuel consumption efficiency of the vehicle. Such a drive mode is also referred to as an EV (Electric Vehicle) drive mode.
In order to improve drive performance in this EV drive mode, it is preferable to increase the charging/discharging performance of the battery pack. Since battery packs deteriorate due to over-discharged operation, over-charged operation, and the like, it is important that battery packs are held within a proper range of remaining capacity (also referred to as State Of Charge (SOC), battery charge amount, charge state value and the like, and hereinafter occasionally referred to as “SOC”).
Japanese Patent Laid-Open Publication No. 2008-312381 proposes a method that improves the estimation accuracy of SOC of a storage battery. According to this method, as shown in FIG. 8, first, a vehicle starts running in an EV drive mode (time t1).
In the EV drive mode, when a reset request for a first power storage unit BAT1 (SOC1 reset request) is issued, current control is performed to discharge actively the first power storage unit BAT1. After time t2 when charging by an external power supply becomes allowed, a discharge current of the first power storage unit BAT1 to be reset is kept at a given current value, and a second power storage unit BAT2 not to be reset is charged with a charge current including at least the discharge current of the first power storage unit BAT1. When a distinctive point appears on a battery voltage characteristic of the first power storage unit BAT1 at a subsequent time t3, an estimated value of an SOC of the first power storage unit BAT1 is reset to a predetermined reference value. This control can improve the estimation accuracy of SOC of the power storage unit.
However, estimation accuracy of SOC is limited. For this reason, switching accuracy between BEV and HEV drive modes in accordance only with calculated SOC is limited. In particular, even if an error between the calculated value of SOC and actual SOC is small, errors are accumulated over a long period of use. Accordingly, the accumulated error will be large. If accumulated error of the calculated values of SOC becomes large, precise control will be difficult. Imprecise charging/discharging operation control may affect the battery pack. This increases the risk of battery life deterioration.
The present invention is aimed at solving the problem. It is a main object of the present invention to provide a power supply device capable of changing a charging/discharging current limit values at appropriate timing whereby reducing the adverse effect on a battery pack.